Generally, there exist various fluid jet methods by which a fluid, such as ink, is ejected onto a target (printing medium). Here, the following will explain an ink jet printing method in which the ink is used as the fluid.
As drop on demand ink jet printing methods, (i) a piezo printing method in which a piezoelectric phenomenon is utilized, (ii) a thermal printing method in which a film boiling phenomenon of ink is utilized, and (iii) an electrostatic attraction printing method in which an electrostatic phenomenon is utilized, etc are developed. Especially, in recent years, a high-resolution ink jet printing method is strongly demanded. To realize the high-resolution ink jet printing, it is indispensable to reduce the size of the ink droplet to be ejected.
Here, the movement of the ink droplet, which is ejected from the nozzle and lands on the printing medium, is expressed by a motion equation (Equation (1)) below.
                              ρ          ⁢                                          ⁢                      ink            ·                          (                                                4                  /                  3                                ·                π                ·                                  d                  3                                            )                                ⁢                                    ⅆ              v                        /                          ⅆ              t                                      =                              -            Cd                    ·                      (                                                            1                  /                  2                                ·                ρ                            ⁢                                                          ⁢                              air                ·                                  v                  2                                                      )                    ·                      (                          π              ·                                                d                  2                                /                4                                      )                                              (        1        )            
The above ρink is a volume density of ink, V is a volume of a droplet, v is a velocity of a droplet, Cd is a drag coefficient, ρair is an air density, and d is a radius of an ink droplet. Cd is expressed by Equation (2) below.Cd=24/Re·(1+3/16·Re0.62)  (2)
Re in Equation (2) is a Reynolds number. Re is expressed by Equation (3) below.Re=2·d·ρink·v/η  (3)In Equation (3), η is an air viscosity.
The influence exercised by the radius of the droplet on the movement energy of the ink droplet of the left side of Equation (1) is greater than the influence exercised by the radius of the droplet on the viscous resistance of the air of the right side of Equation (1). On this account, when the velocity of the droplet is constant, the smaller the droplet becomes, the more quickly the velocity of the droplet decreases. As a result, the droplet may not be able to reach the printing medium separated by a predetermined distance, or the positioning accuracy of the droplet is low even when the droplet reaches the printing medium.
To prevent these from occurring, it is necessary to increase an initial velocity of the ejected droplet, that is, it is necessary to increase an ejection energy per unit volume.
However, according to the conventional piezo ink jet head and the conventional thermal ink jet head, Problems (A) to (C) below occur when the size of the ejected droplet is decreased, that is, when the ejection energy of the droplet per unit volume is increased. On this account, it was especially difficult to set the amount of the ejected droplet to be equal to or less than 1 pl, that is, difficult to set the diameter (hereinafter referred to as “droplet diameter”) of the droplet to be equal to or less than Φ10 μm.
Problem (A): The ejection energy of the piezo ink jet head relates to the amount of displacement and a developed pressure of a piezoid to be driven. The amount of displacement of the piezoid inseparably relates to the amount of the ink ejected, that is, to the size of the ink droplet. To reduce the size of the droplet, it is necessary to reduce the amount of displacement. On this account, it is difficult to improve the ejection energy, per unit volume, of the ejected droplet.
Problem (B): The thermal ink jet head utilizes the film boiling phenomenon of ink. Pressure generated when bubbles are formed is physically limited. Moreover, the ejection energy of the ink is substantially determined by the area of a heating element. The area of the heating element is substantially in proportion to a volume of the bubble formed, that is, in proportion to the amount of ink ejected. On this account, by decreasing the size of the ink droplet, the volume of the bubble formed is decreased. In proportion to this decrease, the ejection energy is also decreased. Therefore, it is difficult to improve the ejection energy, per unit volume, of the ejected droplet of the ink.
Problem (C): In both the piezo printing method and the thermal printing method, how much the drive element (heating element) works relates closely to the amount of ink ejected. Therefore, in the case of ejecting extremely fine droplets, it is very difficult to suppress variations in size of the droplets.
Here, as a method for solving the above problems, a method for ejecting fine droplets by using the electrostatic attraction printing method has been developed.
In the electrostatic attraction printing method, a motion equation of the ink droplet ejected from the nozzle is expressed as Equation (4) below.
                                                                        ρ                ⁢                                                                  ⁢                                  ink                  ·                                      (                                                                  4                        /                        3                                            ·                      π                      ·                                              d                        3                                                              )                                                  ⁢                                                      ⅆ                    v                                    /                                      ⅆ                    t                                                              =                            ⁢                                                q                  ·                  E                                -                                  Cd                  ·                                      (                                                                                            1                          /                          2                                                ·                        ρ                                            ⁢                                                                                          ⁢                                              air                        ·                                                  v                          2                                                                                      )                                    ·                                                                                                                      ⁢                              (                                  π                  ·                                                            d                      2                                        /                    4                                                  )                                                                        (        4        )            In Equation (4), q is the amount of electric charge of a droplet, and E is a peripheral electric field intensity.
According to Equation (4), in the electrostatic attraction printing method, the ejected droplet receives, in addition to the ejection energy, an electrostatic force while the droplet is flying. Therefore, it is possible to reduce the ejection energy per unit volume and possible to apply the method to the ejection of a fine droplet.
As an ink jet device using such an electrostatic attraction printing method (hereinafter referred to as “electrostatic attraction ink jet device”), Japanese Unexamined Patent Publication No. 238774/1996 (Tokukaihei 8-238774, Document 1) discloses an ink jet device in which an electrode for applying voltages is provided inside the nozzle. Moreover, Japanese Unexamined Patent Publication No. 127410/2000 (Tokukai 2000-127410, Document 2) discloses an ink jet device which has a slit as a nozzle, is provided with a stylus electrode projected from the nozzle, and ejects ink containing fine particles.
Referring to FIG. 21, the following will explain the ink jet device disclosed in Document 1. FIG. 21 is a cross sectional view schematically showing the ink jet device.
In FIG. 21, reference numeral 101 indicates an ink jet chamber, reference numeral 102 indicates ink, reference numeral 103 indicates an ink chamber, reference numeral 104 indicates a nozzle hole, reference numeral 105 indicates an ink tank, reference numeral 106 indicates an ink supplying path, reference numeral 107 indicates a rotating roller, reference numeral 108 indicates a printing medium, reference numeral 110 indicates a control element portion, and reference numeral 111 indicates a process control portion.
Further, reference numeral 114 indicates an electrostatic field applying electrode portion which is provided in the ink chamber 103 of the ink jet chamber 101, reference numeral 115 indicates a counter electrode portion which is a metallic drum provided at the rotating roller 107, and reference numeral 116 indicates a bias power supply portion for applying a negative voltage of thousands of volts to the counter electrode portion 115. Reference numeral 117 indicates a high voltage power supply portion for supplying a high voltage of hundreds of volts to the electrostatic field applying electrode portion 114, and reference numeral 118 indicates a ground portion.
Here, between the electrostatic field applying electrode portion 114 and the counter electrode portion 115, the negative voltage of thousands of volts applied from the bias power supply portion 116 to the counter electrode portion 115 and a high voltage of hundreds of volts from the high voltage power supply portion 117 are superimposed. In this way, a superimposed electric field is generated. The ejection of the ink 102 ejected from the nozzle hole 104 is controlled by means of the superimposed electric field. In addition, reference numeral 119 indicates a projected meniscus which is formed at the nozzle hole 104 by the bias voltage of thousands of volts applied to the counter electrode portion 115.
The following will explain operations of the electrostatic attraction ink jet device configured as above.
First, the ink 102 in the ink tank 105 passes through the ink supplying path 106 by the capillary phenomenon, and is transferred to the nozzle hole 104 of the ink jet chamber 101. At this time, the printing medium 108 is mounted on a surface of the counter electrode portion 115 provided face to face with the nozzle hole 104, and the surface is opposed to the nozzle hole 104.
The ink 102 having reached the nozzle hole 104 forms the projected ink meniscus 119 by the bias voltage of thousands of volts applied to the counter electrode portion 115. Moreover, a signal voltage of hundreds of volts is applied from the high voltage power supply portion 117 to the electrostatic field applying electrode portion 114 which is provided in the ink chamber 103. The signal voltage thus applied is superimposed on the voltage applied from the bias power supply portion 116 to the counter electrode portion 115. Then, by the superimposed electric field, the ink 102 is ejected onto the printing medium 108. As a result, a printed image is formed.
Next, referring to FIGS. 22(a) to 22(c), the following will explain the movement of the meniscus, until the droplet is ejected, of the droplet of the ink jet device disclosed in Document 1.
As shown in FIG. 22(a), before a drive voltage is applied, a projected meniscus 119a is formed on the surface of the ink at the nozzle hole 104 because of the balance between (i) the electrostatic force of the bias voltage applied to the ink and (ii) the surface tension energy of the ink.
As shown in FIG. 22(b), when the drive voltage is applied, the electric charge generated on the fluid surface starts concentrating on the center of the fluid surface. As a result, a meniscus 119b is so formed that the center of the fluid surface is highly projected.
As shown in FIG. 22(c), when the drive voltage is continuously applied, the electric charge generated on the fluid surface further concentrates on the center of the fluid surface. This results in the formation of a meniscus 119c which is a semilunar shape called “taylor cone”. When the electrostatic force of the electric charge concentrated on the top of the taylor cone exceeds the surface tension energy of the ink, a droplet is formed and ejected.
Next, referring to FIG. 23, the following will explain the ink jet device disclosed in Document 2. FIG. 23 is a diagram showing a schematic configuration of the ink jet device.
As shown in FIG. 23, a holding member of the present ink jet device contains (i), as an ink jet head, a line-shaped printing head 211 formed by using low dielectric materials (acrylic resin, ceramics, etc.), (ii) a counter electrode 210 which is made of metal or high dielectric materials and is provided face to face with an ink-ejecting opening of the printing head 211, (iii) an ink tank 212 for storing ink which is made by dispersing electrified pigment particles in nonconductive ink medium, (iv) ink circulating system (pumps 214a and 214b, pipings 215a and 215b) for circulating ink between the ink tank 212 and the printing head 211, (v) a pulse voltage generating device 213 which applies a pulse voltage, for ejecting an ink droplet which forms one pixel of a record image, to each ejection electrode 211a, (vi) a drive circuit (not shown) which controls the pulse voltage generating device 213 in accordance with image data, (vii) a printing medium feeding apparatus (not shown) which causes a printing medium A to pass through a space between the printing head 211 and the counter electrode 210, (viii) a controller (not shown) which controls the device entirely, etc.
The ink circulating system is composed of (i) two pipings 215a and 215b each of which connects the printing head 211 with the ink tank 212 and (ii) two pumps 214a and 214b which are driven by the controller.
The ink circulating system is divided into (i) an ink supplying system which supplies ink to the printing head 211 and (ii) an ink catching system which catches ink from the printing head 211.
In the ink supplying system, the ink is pumped up by the pump 214a from the ink tank 212, and the ink thus pumped up is delivered to the ink supplying portion of the printing head 211 through the piping 215a. Meanwhile, in the ink catching system, the ink is pumped up by the pump 214b from the a catching portion of the printing head 211, and the ink thus pumped up is compulsorily caught in the ink tank 212 through the piping 215b. 
Moreover, as shown in FIG. 24, the printing head 211 includes (i) an ink supplying portion 220a which spreads the ink, supplied from the piping 215a of the ink supplying system, so that the ink is spread to be as wide as a line, (ii) an ink flow path 221 which guides the ink, supplied from the ink supplying portion 220a, so that the ink forms a mountain-shape, (iii) an ink collecting portion 220b which connects the ink flow path 221 with the piping 215b of an ink collecting system, (iv) a slit-shaped ink-ejecting opening 222 which is open to the counter electrode 210 at the mountaintop of the ink flow path 221 and has an appropriate width (approximately 0.2 mm), (v) a plurality of ejection electrodes 211a which are provided in the ink ejection opening 222 with a predetermined pitch (approximately 0.2 mm), and (vi) party walls 223 which are made of low dielectric materials (for example, ceramic) and are provided on both sides and an upper surface of each ejection electrode 211a. 
Each of the ejection electrodes 211a is made of metals, such as copper, nickel, etc. On the surface of the ejection electrode 211a, a low dielectric film (for example, polyimide film), which excels in wettability, for preventing pigments from being adhered is formed. Moreover, the top of each ejection electrode 211a is formed like a triangular pyramid. Each ejection electrode 211a projects from the ink-ejecting opening 222 toward the counter electrode 210 by an appropriate length (70 μm to 80 μm).
In the above configuration, in accordance with control by the controller, the above-described drive circuit (not shown) gives a control signal to the pulse voltage generating device 213 during a time corresponding to gradation data included in the image data. Then, the pulse voltage generating device 213 superimposes a pulse Vp, whose pulse top corresponds to the kind of the control signal, on the bias voltage Vb so as to output as a high voltage signal a pulse voltage thus superimposed.
When the image data is transferred, the controller drives two pumps 214a and 214b of the ink circulating system. Then, the ink is delivered from the ink supplying portion 220a, and the negative pressure is applied to the ink collecting portion 220b. The ink flowing in the ink flow path 221 passes through the gap between the party walls 223 by the capillary phenomenon. Then, the ink spreads so as to reach the top of each ejection electrode 211a. At this time, the negative pressure is applied to the surface of each ink fluid near the top of the ejection electrode 211a. Therefore, the ink meniscus is formed on the top of each ejection electrode 211a. 
Further, the controller controls the printing medium feeding apparatus so that the printing medium A is fed in a predetermined direction. Moreover, the controller controls the drive circuit so that the above-described high voltage signal is applied between the printing medium A and the ejection electrode 211a. 
Referring to FIGS. 25 to 28, the following will explain the movement of the meniscus, until the droplet is ejected, of the droplet of the ink jet device disclosed in Document 2.
As shown in FIG. 25, when the pulse voltage generated by the pulse voltage generating device 213 is applied to the ejection electrode 211a in the printing head 211, an electric field, which goes from the ejection electrode 211a to the counter electrode 210, is generated. Here, because the ejection electrode 211a whose top is sharp is used, the strongest electric field is generated around the top of the ejection electrode 211a. 
As shown in FIG. 26, when such an electric field is generated, each electrified pigment particle 201a in the ink solvent moves toward the surface of the ink fluid by the force fE (FIG. 25) exerted from the electric field. In this way, the density of pigment around the surface of the ink fluid is increased.
As shown in FIG. 27, when the density of pigment is thus increased, a plurality of electrified pigment particles 201a around the surface of the ink fluid starts cohering at the opposite side of the electrode. Then, a pigment aggregate 201 starts growing to form a spherical shape near the surface of the ink fluid. Then, the electrostatic repulsive force fcon from the pigment aggregate 201 starts influencing each electrified pigment particle 201a. That is, each electrified pigment particle 201a is influenced by the total force ftotal which is a resultant force of the electrostatic repulsive force fcon from the pigment aggregate 201 and the force fE from the electric field E generated by the pulse voltage.
Therefore, in the case in which the electrostatic repulsive force between the electrified pigment particles does not excess the force of cohesion of the electrified pigment particles, when the force fE exceeds the electrostatic repulsive force fcon (fE≧fcon), the electrified pigment particles 201a form the pigment aggregate 201. Note that, the force fE is applied from the electric field to the electrified pigment particle 201a (electrified pigment particle 201a which is located on a straight line between the top of the ejection electrode 211a and the center of the pigment aggregate 201) to which the total force ftotal in a direction of the pigment aggregate 201 is applied.
The pigment aggregate 201 formed by n pieces of electrified pigment particles 201a receives an electrostatic repulsive force FE from the electric field E generated by the pulse voltage, and also receives the binding force Fesc from the ink solvent. When the electrostatic repulsive force FE and the binding force Fesc are balanced, the pigment aggregate 201 becomes stable in a state in which the pigment aggregate 201 projects slightly from the surface of the ink fluid.
Further, as shown in FIGS. 28(a) to 28(c), when the pigment aggregate 201 grows and the electrostatic repulsive force FE exceeds the binding force Fesc, the pigment aggregate 201 is separated from the surface 200a of the ink fluid.
Incidentally, according to the principle of the conventional electrostatic attraction printing method, the meniscus is projected by concentrating the electric charge on the center of the meniscus. The curvature radius of a taylor cone tip portion thus projected is determined by the amount of concentrated electric charge. When the electrostatic force of the amount of concentrated electric charge and the electric field intensity exceeds the surface tension energy of the meniscus, the droplet starts to be ejected.
The maximum amount of electric charge of the meniscus is determined by the physical-property value of the ink and the curvature radius of the meniscus. Therefore, the minimum size of the droplet is determined by the physical-property value of the ink (especially, the surface tension energy) and the intensity of the electric field generated at the meniscus portion.
Generally, the surface tension energy tends to become lower in a fluid containing solvents than in a pure solution. Because typical ink contains various solvents, it is difficult to increase the surface tension energy. On this account, the ink surface tension energy is considered to be constant, and a method of decreasing the size of the droplet by increasing the electric field intensity is used.
Therefore, according to the principle of the ejection of the ink jet device disclosed in each of Documents 1 and 2, an electric field whose intensity is high is generated at the meniscus region whose area is much larger than a project area of the ejected droplet. By the field, the electric charge is concentrated on the center of the meniscus. Then, by an electrostatic force of the concentrated electric charge and the electric field, the ejection is carried out. Therefore, it is necessary to apply an extremely high voltage of about 2,000 V. On this account, it is difficult to control the driving, and there is a problem in view of the safety of the operation of the ink jet device.
(Document 1)
Japanese Unexamined Patent Publication No. 238774/1996 (Tokukaihei 8-238774, published on Sep. 17, 1996)
(Document 2)
Japanese Unexamined Patent Publication No. 127410/2000 (Tokukai 2000-127410, published on May 9, 2000)
(Document 3)
Japanese Unexamined Patent Publication No. 31757/1983 (Tokukaisho 58-31757, published on Feb. 24, 1983)
(Document 4)
Japanese Unexamined Patent Publication No. 189548/1992 (Tokukaihei 4-189548, published on Jul. 8, 1992)
(Document 5)
Japanese Unexamined Patent Publication No. 268304/1999 (Tokukaihei 11-268304, published on Oct. 5, 1999)
To increase the electric field intensity without applying a high voltage, it is necessary to reduce the width or diameter of a portion (ejection starting portion) from which an ink droplet is ejected. With this, it is possible to decrease the size of the electric field which is conventionally large, and it is also possible to drastically reduce the voltage required for the movement of the electric charge, that is, the voltage required for applying to the fluid the electric charge, the amount of which is such that the fluid is electrostatically attracted. Moreover, when the diameter of the fluid-ejecting hole of the nozzle is Φ8 μm or less, the intensity distribution of the electric field concentrates near an ejecting surface of the fluid-ejecting hole. Moreover, the change in the distance between the counter electrode and the fluid-ejecting hole of the nozzle does not influence the intensity distribution of the electric field any more. On this account, it is not necessary to apply a high voltage of 2,000 V which is conventionally necessary. As a result, it is possible to improve safety when using a fluid jet device.
Moreover, because it is possible to reduce the area of the electric field as described above, it becomes possible to generate a high electric field in a small area. As a result, it becomes possible to form fine droplets. On this account, when the droplet is ink, it is possible to realize a high resolution printed image.
Furthermore, because the region where the electric charge is concentrated and the meniscus region of the fluid become substantially the same in size, the amount of time for the electric charge to move in the meniscus region does not influence the response of ejection. As a result, it is possible to improve the velocity of the ejected droplet (print speed when the droplet is an ink).
However, the ink flow path becomes narrow in the case of reducing in size the ejection starting portion (nozzle hole). Therefore, if an ink jet device is left with ink therein, the ink dehydrates and solidifies, or particles in a solution aggregates. This causes clogging of the nozzle hole. Moreover, since an aggregate solidifies easily, the aggregate sticks to an inner surface of the ink flow path. This reduces the cross sectional area of the flow path. Therefore, an ink supply to the ejection starting portion becomes unstable. Thus, the ejection becomes unstable. The clogging or unstable ejection is a major factor for fluctuating the size of the dot formed, causing defects, or decreases the image quality.
Therefore, a method for preventing the clogging or removing the clogging is necessary. The method for preventing the clogging is exemplified by a method for supplying solvent vapor (for example, Tokukaisho 58-31757) and a method for washing (for example, Tokukaihei 4-189548). The method for supplying solvent vapor cannot deal with the clogging caused in the case in which a multichannel ejection head is used and a specific nozzle is not used for a long period of time. Moreover, in the case of the method for washing, it is difficult to wash a head since the head has a small ejection diameter.
Meanwhile, the method for removing the clogging is exemplified by a method for applying a high voltage at a maintenance portion to cause the clogged ink to be ejected (Tokukaihei 11-268304). The following will explain this method in reference to FIG. 29. FIG. 29 is a diagram showing a schematic configuration of an ink jet printing device.
The ink jet printing device includes: a printing head 305 supported by a supporting axis 306; a counter electrode 301 which is opposed to the printing head 305 and holds a printing sheet 302; a purging head 307 provided at a position adjacent to the counter electrode 301; and moving means for causing the printing head 305 to move to a drawing position and a position opposed to the purging head 307. If, in this ink jet printing device, an adhered substance adheres to an ink ejecting portion of the printing head 305 and the printing head 305 is clogged, it is possible to carry out a purging of the printing head 305 in the following manner.
That is, the printing head 305 is moved along the supporting axis 306 from a position in front of the counter electrode 301 to a position opposed to the purging head 307. In this state, between the printing head 305 and the purging head 307, an electric field stronger than an electric field generated when forming a printing dot is generated. With this, an ink droplet is ejected toward the purging head 307 by a stronger electrostatic force. This makes it possible to remove the adhered substance from the ink ejecting portion of the printing head 305.
However, according to the method disclosed in Document 5, it is necessary to move the printing head 305 back to a drawing place after removing the clogging. If the time necessary for this moving back is long, the clogging may occur again, for example, before starting the drawing. On this account, the drawing can be carried out only with respect to a cylindrical printing medium 302 since the time necessary for this moving back is short in this case, and it is difficult to carry out the drawing with respect to a flat medium since the time necessary for this moving back is long in this case. Further, the ejection cannot be carried out in the case of using ink made of a substance which dehydrates in a short period of time, such as ink which dehydrates while the printing head 305 is moving. Moreover, due to, for example, an increase in viscosity of an ejected substance (ink), it is impossible to suppress variations of the amount of ejected ink in an initial ejection.
The present invention was made to solve the above-described problems, and an object of the present invention is to provide electrostatic attraction fluid ejecting method and apparatus which (i) can quickly remove the clogging of an ejection head with a nozzle provided at any position, (ii) cause less variations in an initial ejection and (iii) have high reliability of ejection, in a configuration capable of ejecting fluid by using an electrostatic force.